Make Pi the default agent provider

[LasmarKhalifa]

Closes #997

Makes Pi the built in default agent provider and applies doc updates to reflect that change.

[LasmarKhalifa]

• Add env variable override for the default agent provider #999
• Make Pi the default agent provider #996 👈 (View in Graphite)
• main

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[semgrep-pr-bot]

⚠️ Deprecated LLM Models Detected

This PR contains references to 3 deprecated model(s). Consider migrating to the recommended replacements.

⚠️ gpt-4o-mini (azure)

• Occurrences: 2
• Severity: WARNING
• Provider Shutdown: 2026-10-01
• Provider Suggested Replacements: gpt-4.1-mini

Locations:

• tutorial/02_chaining_cogs/code_review.rb:16
• tutorial/02_chaining_cogs/session_resumption.rb:8

Suggested Action: Consider migrating to gpt-4.1-mini

---

⚠️ gpt-4o-mini (openai)

• Occurrences: 2
• Severity: WARNING
• Shopify Replacement: gpt-4.1-mini
• Hotswap Date: 2026-05-27

Locations:

• tutorial/02_chaining_cogs/code_review.rb:16
• tutorial/02_chaining_cogs/session_resumption.rb:8

Recommended Action: Replace with gpt-4.1-mini before 2026-05-27

---

⚠️ gpt-5.4-nano (azure)

• Occurrences: 1
• Severity: WARNING
• Provider Shutdown: 2027-03-18

Locations:

• tutorial/02_chaining_cogs/session_resumption.rb:13

---

📚 Resources

• OpenAI Proxy Deprecations
• Deprecations.info

💡 This is an informational warning - it will not block your PR. However, please plan to migrate away from deprecated models before their shutdown dates.
💡 Please reach out to #help-proxy-shopify-ai if you have any questions.

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